Position sensor system

ABSTRACT

A sensing mechanism includes a magnetic source, a magnetic flux sensor, a sensor backing on which the magnetic source and flux sensor are mounted, and a ferromagnetic target, where the magnetic source, magnetic flux sensor, and ferromagnetic target are positioned to form a magnetic circuit from the magnetic source to the target, from the target to the sensor, and returning to the magnetic source through the sensor backing.

This application claims the benefit of U.S. Provisional Patentapplication 60/946,686, entitled: POSITION FEEDBACK FOR SELF BEARINGMOTOR, filed 27 Jun. 2007, which is incorporated by reference herein inits entirety.

This application is related to U.S. patent application Ser. No.11/769,688, entitled: COMMUTATION OF AN ELECTROMAGNETIC PROPULSION ANDGUIDANCE SYSTEM, filed 27 Jun. 2007, U.S. patent application Ser. No.11/769,651, entitled: REDUCED-COMPLEXITY SELF-BEARING BRUSHLESS DCMOTOR, filed 27 Jun. 2007, U.S. Provisional Patent application60/946,693, entitled: MOTOR STATOR WITH LIFT CAPABILITY AND REDUCEDCOGGING CHARACTERISTICS, filed 27 Jun. 2007, and U.S. Provisional Patentapplication 60/946,687, entitled “ROBOT DRIVE WITH MAGNETIC SPINDLEBEARINGS”, filed 27 Jun. 2007, all of which are incorporated byreference herein in their entirety.

BACKGROUND

The presently disclosed embodiments are directed to determiningposition, in particular to providing non-contact and non-invasiveposition determination for a motor.

BRIEF DESCRIPTION OF RELATED DEVELOPMENTS

Motor systems may require measurements of eccentricity and orientationof a reactive element, such as a rotor, in order to maintain a desiredgap between a stator and the reactive element to produce a desiredamount of motive force, axial and radial stiffness, and to properlycontrol motion of the reactive element. For example, in a self bearingmotor, the gap information may typically be obtained from proximitysensors that may detect gaps between the stator and the rotor at variouslocations. The proximity sensors are often complemented by anothermeasurement device, such as a position resolver, which determines theorientation of the rotor with respect to the stator.

In certain applications, materials must be processed in a controlled,clean atmosphere where microscopic contaminates may represent a severeproblem. In those applications, cleanliness may be directly related toyield, which may in turn affect cost. Other applications may includeprocessing steps utilizing hostile atmospheres of highly corrosive gasesand high temperatures. Motors with contact bearings may wear, produceparticulate contamination, and eventually fail due to the hostileenvironment. Bearings may also exhibit an unacceptable amount ofvibration and play before failing. While self-bearing motors may providea viable alternative for these applications, it would be undesirable topenetrate or invade the hostile environment with cables or otherconductors in order to measure the exact position of the reactiveelement. Optical techniques may also be disadvantageous because they mayrequire a “window” into the hostile environment that may likewisecompromise the integrity of an enclosure containing the environment.

It would be advantageous to provide a system of sensors and scalesattached to a motor reactive element such as a rotor to provide accurateposition and eccentricity measurements.

It would also be advantageous to have a sensor system that utilizesmagnetic flux density in order to accurately measure the position of arotor and to accurately measure the scales that may be attached to orintegral with the reactive element.

It would be also be advantageous to have a motor feedback system thatsimultaneously measures the eccentricity and orientation of the rotorwith respect to the stator without using two types of sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the disclosed embodimentsare explained in the following description, taken in connection with theaccompanying drawings, wherein:

FIGS. 1A and 1B show schematic diagrams of exemplary motors suitable forpracticing the disclosed embodiments;

FIG. 2 shows an exemplary robot transport in which the exemplaryembodiments may be utilized;

FIG. 3 shows an exemplary substrate processing apparatus in which theexemplary embodiments may be utilized;

FIG. 4 shows a schematic diagram of a self bearing motor utilizing theexemplary embodiments;

FIG. 5 shows an exemplary sensing mechanism according to the disclosedembodiments;

FIG. 6 shows a magnetic circuit equivalent of the sensing mechanism ofFIG. 5;

FIG. 7 shows an exemplary incremental scale;

FIG. 8 shows another exemplary sensor system embodiment;

FIGS. 9A and 9B show additional incremental scale exemplary embodiments;

FIG. 10 shows a Gray code pattern;

FIG. 11 shows an example of a single scale for indicating absoluteposition;

FIG. 12 illustrates an exemplary sensor output change;

FIG. 13 shows an exemplary embodiment with multiple scales located onthe same diameter;

FIG. 14 shows a multiple sensor system;

FIG. 15 shows another exemplary sensor system suitable for use with theembodiments described;

FIG. 16 shows an exemplary arrangement of magnetic sensors around aferromagnetic element;

FIG. 17 illustrates a portion of a drive section incorporating aspectsof an exemplary embodiment;

FIG. 18 is a schematic illustration of a feedback system in accordancewith an exemplary embodiment;

FIG. 19 is a schematic illustration of a feedback system in accordancewith an exemplary embodiment;

FIGS. 20A and 20B show additional embodiments for providing displacementmeasurement of a rotor using a plurality of sensor sets; and

FIG. 21 shows an embodiment where sensor systems are used to read ascale.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Although the presently disclosed embodiments will be described withreference to the drawings, it should be understood that they may beembodied in many alternate forms. It should also be understood that Inaddition, any suitable size, shape or type of elements or materialscould be used.

The exemplary embodiments are directed to position sensing systems formotors that provide a desired level of accuracy and repeatability.Additional embodiments include systems for use with motors in hostile orclean environments, in particular, robot drive applications where therotor and stator may be atmospherically isolated from each other.

FIG. 1A shows a schematic diagram of an exemplary motor 10 suitable forpracticing the embodiments disclosed herein. Although the presentlydisclosed embodiments will be described with reference to the drawings,it should be understood that they may be embodied in many alternateforms. It should also be understood that any suitable size, shape ortype of elements or materials could be used.

Motor 10 includes an reactive element 11, in this embodiment in the formof a rotor, winding sets 12, 15, and a stator 14. The embodiment ofexemplary motor 10 depicted in FIG. 1 is shown as having a rotaryconfiguration, although other embodiments may include linearconfigurations as will be described below. Rotor 11 may have anysuitable construction. The rotor 11 may have one or more magneticsources mounted thereon, for example, permanent magnets, electromagnetsor other types of magnetic sources. Winding sets 12, 15 may include oneor more windings and may be driven by current amplifier 25 which mayinclude software, hardware, or a combination of software and hardwaresuitable for driving the winding sets. The current amplifier 25 may alsoinclude a processor 27, a commutation function 30 and a current loopfunction 35 for driving the winding sets. The commutation function 30may supply current for one or more windings of each winding setaccording to a set of specified functions, while the current loopfunction 35 may provide a feedback and driving capability formaintaining the current through the windings as supplied. The processor27, commutation function 30, and current loop function 35 may alsoinclude circuitry for receiving feedback from one or more sensors orsensor systems that provide position information. Each current amplifierdisclosed herein includes circuitry, hardware or software in anycombination as required to perform the functions and computations forthe disclosed embodiments.

FIG. 1B shows another exemplary embodiment having a linearconfiguration. Motor 20 includes an reactive element 21, in thisembodiment having the form of a platen, winding sets 22, 24 and a stator45. Similar to the embodiment of FIG. 1, platen 21 may have one or moremagnetic sources mounted thereon, for example, permanent magnets,electromagnets or other types of magnetic sources. Platen 21 may beconstructed in any suitable manner and winding sets 22, 24 may includeone or more windings.

Both motors 10, 20 may utilize a minimal air gap and ferromagneticmaterials to affect a substantial gain in the magnetic flux densityacross the air gap which in turn produces desired axial and tiltstiffness. It would be advantageous to precisely measure the position ofthe reactive element of the motors 10, 20.

An exemplary robot transport 200 is shown in FIG. 2. The transport mayinclude at least one arm having an upper arm 210, a forearm 220 and atleast one end effector 230. The end effector may be rotatably coupled tothe forearm and the forearm may be rotatably coupled to the upper arm.The upper arm may be rotatably coupled to, for example a drive section240 of the transport apparatus which may include one or more of motors10, 20 above.

An exemplary substrate processing apparatus 300 is shown in FIG. 3incorporating features of the exemplary embodiments. In this example theprocessing apparatus 300 is shown as having a general batch processingtool configuration. In alternate embodiments the tool may have anydesired arrangement, for example the tool may be configured to performsingle step processing of substrates. In other alternate embodiments,the substrate apparatus may be of any desired type such as sorter,stocker, metrology tool, etc. The substrates 215 processed in theapparatus 100 may be any suitable substrate including, but not limitedto, liquid crystal display panels, semiconductor wafers, such as a 200mm, 300 mm, 450 mm wafers or any other desired diameter substrate, anyother type of substrate suitable for processing by substrate processingapparatus 100, a blank substrate, or an article having characteristicssimilar to a substrate, such as certain dimensions or a particular mass.

In this embodiment, apparatus 300 may generally have a front section105, for example forming a mini-environment and an adjoiningatmospherically isolatable section 110, which for example may beequipped to function as a vacuum chamber. In alternate embodiments, theatmosphere isolated section may hold an inert gas (e.g. N2) or any otherisolated and/or controlled atmosphere.

In the exemplary embodiment, front section 105 may generally have, forexample one or more substrate holding cassettes 115, and a front endrobot arm 120, similar to that shown in FIG. 2. The front section 105may also, for example, have other stations or sections such as analigner 162 or buffer located therein. Section 110 may have one or moreprocessing modules 125, and a vacuum robot arm 130 which also may besimilar to that shown in FIG. 2. The processing modules 125 may be ofany type such as material deposition, etching, baking, polishing, ionimplantation cleaning, etc.

As may be realized the position of each module, with respect to adesired reference frame, such as the robot reference frame, may beregistered with controller 170. Also, one or more of the modules mayprocess the substrate(s) 195 with the substrate in a desiredorientation, established for example using a fiducial (not shown) on thesubstrate. Desired orientation for substrate(s) in processing modulesmay also be registered in the controller 170. Vacuum section 110 mayalso have one or more intermediate chambers, referred to as load locks.

The embodiment shown in FIG. 3 may have two load locks, load lock A 135,and load lock B 140. Load locks A and B operate as interfaces, allowingsubstrates to pass between front section 105 and vacuum section 110without violating the integrity of any vacuum that may be present invacuum section 110. Substrate processing apparatus 100 generallyincludes a controller 170 that controls the operation of substrateprocessing apparatus 100. Controller 170 has a processor and a memory178. In addition to the information noted above, memory 178 may includeprograms including techniques for on-the-fly substrate eccentricity andmisalignment detection and correction. Memory 178 may further includeprocessing parameters, such as temperature and/or pressure of processingmodules, and other portions or stations of sections 105, 110 of theapparatus, temporal information of the substrate(s) 215 being processedand metric information for the substrates, and program, such asalgorithms, for applying this ephemeris data of apparatus and substratesto determine on the fly substrate eccentricity.

In exemplary apparatus 300, front end robot arm 120, also referred to asan ATM robot, may include a drive section 150 and one or more arms 155.At least one arm 155 may be mounted onto drive section 150 which mayinclude one or more motors similar to those of FIGS. 1A and 1B above. Atleast one arm 155 may be coupled to a wrist 160, which in turn iscoupled to one or more end effector(s) 165 for holding one or moresubstrate(s) 215. End effector(s) 165 may be rotatably coupled to wrist160. ATM robot 120 may be adapted to transport substrates to anylocation within front section 105. For example, ATM robot 120 maytransport substrates among substrate holding cassettes 115, load lock A135, and load lock B 140. ATM robot 120 may also transport substrates215 to and from the aligner 162. Drive section 150 may receive commandsfrom controller 170 and, in response, direct radial, circumferential,elevational, compound, and other motions of ATM robot 120.

In the exemplary embodiment, vacuum robot arm 130 may be mounted incentral chamber 175 of section 110. Controller 170 may operate to cycleopenings 180, 185 and coordinate the operation of vacuum robot arm 130for transporting substrates among processing modules 125, load lock A135, and load lock B 140. Vacuum robot arm 130 may include a drivesection 190 and one or more end effectors 195. In other embodiments, ATMrobot 120 and vacuum robot arm 130 may be any suitable type of transportapparatus, for example, a SCARA-type robot, an articulating arm robot, afrog leg type apparatus, or a bi-symmetric transport apparatus.

Referring now to FIG. 4 a schematic diagram of a self bearing motor 400is shown that may be employed in, for example, drive section 240 oftransport robot 200. Self bearing motor 400 includes a rotor 410 and astator 415. A single rotor/stator combination is shown in FIG. 4 forexemplary purposes only and it should be realized that the motor 400 mayinclude any suitable number of rotors having any suitable configuration.In the exemplary embodiment of FIG. 4, the stator 415 may besubstantially similar to, for example, the stator 14 of FIG. 1 describedabove. Correspondingly, the rotor 410 may also be substantially similarto rotor 11 of FIG. 1. Rotor 410 may be constructed of, for example, aferromagnetic material and may include permanent magnets 420 and an ironbacking 425.

In other alternate embodiments the permanent magnets may be replacedwith any suitable ferromagnetic material for interacting with thestator, including other types of magnetic sources, for example,electromagnets. The rotor magnets 420 may include an array of magnetshaving alternating polarities mounted around a periphery of the rotor.The periphery of the rotor may be an internal peripheral wall or anexternal peripheral wall of the rotor. In alternate embodiments themagnets 420 may be embedded within the rotor. In other alternateembodiments, the magnets 420 may be located at any suitable location onor in the rotor 410.

The stator 415 may include windings sets which when energized drive therotor rotationally, radially and/or axially. In this exemplaryembodiment the stator 415 may be constructed of a ferromagnetic materialbut in alternate embodiments the stator may be constructed of anysuitable material (in the case of non magnetic material stator, magneticmaterial may be included in the stator to provide for passivelevitation. The interaction between the stator 415 and the rotor magnets420 may produce passive forces in the direction of arrow 430 thatpassively levitate the rotor 410. Radial or attractive forces may begenerated as a result of the magnetic flux lines 435 in the directionsof for example, arrows 440, 445. These attractive forces may create anunstable condition such that the windings may be energized to activelycenter and/or position the rotor radially to maintain the geometriccenter of the rotor/axis of rotation at a desired location.

It should be noted that in the exemplary embodiment of FIG. 4 the rotor410 is isolated from the stator 415 by a wall 450 that allows the rotor410 to operate in a different environment from the stator 415, forexample, a vacuum. Wall 450 may be constructed of a non-magneticmaterial thus allowing magnetic force to traverse the wall between therotor 410 and stator 415.

Turning now to FIG. 5, the principles of a sensing mechanism 500according to the disclosed embodiments will now be explained. Theembodiment of FIG. 5 shows a ferromagnetic target, for example aferromagnetic backing 510. The ferromagnetic backing may be attached toa reactive motor element, for example, a rotor 505. The rotor 505 mayhave one or more permanent magnets 515. The rotor may be enclosed withina chamber 525 capable of supporting an environment different from thatoutside the chamber, for example a vacuum, high temperature, orcorrosive atmosphere. The chamber 525 may be constructed of anonmagnetic material. The rotor 505 may be driven by one or more coils520 located outside the chamber 525.

The sensing mechanism 500 includes a read head 545 having a magneticsource 530 and a sensor 540 mounted on a sensor backing 550. In thisembodiment, the sensing mechanism implements a magnetic circuit or fluxloop path formed from the magnetic source 530, an air gap 555 betweenthe magnetic source 530 and the ferromagnetic target, in this embodimentrotor backing 510, a path 560 through the rotor backing 510, a returnpath through the air gap 535 to the sensor 540, returning to themagnetic source 530 by way of the sensor backing 550. The magnetic fluxloop path is continuously closed and the sensor 540 is capable ofdetermining the magnetic flux intensity which is dependent on, amongother factors, the distance between the magnetic source 530 and therotor backing 510. In at least one embodiment, the sensor 540 has anoutput that uniquely correlates the magnetic flux intensity with thedistance between the magnetic source and the ferromagnetic target.Magnetic source 530 may include one or more permanent magnets,electromagnets or any other suitable magnetic source. Sensor 540 mayinclude one or more magnetic flux sensors, Hall effect, magnetoresistive, or any other type of sensor suitable for sensing magneticflux.

FIG. 6 shows the magnetic circuit equivalent of the sensor mechanism ofFIG. 5. The magnetic source 530 is represented by a constant flux sourceφr and a magnetic source reluctance Rm in parallel. The intensity of themagnetic flux φ depends on the reluctance of the air gap 555 from themagnetic source 530 to the rotor backing 510 in combination with the airgap 535 from the rotor backing 510 to the sensor 540, represented as2Rg, the magnetic source reluctance Rm, the reluctance of the rotorbacking R_(T) and reluctance of the sensor backing R_(B). The magneticsource reluctance Rm, the reluctance of the rotor backing R_(T) andreluctance of the sensor backing R_(B) may be relatively constant. Theair gap reluctance 2Rg is directly dependent on the distances betweenthe magnetic source 530 and the rotor backing 510 and between the rotorbacking 510 and the sensor 540, and thus may be uniquely correlated withthose distances as they vary. Thus, the position of the rotor backingalong distances 535, 555 may be determined without any invasion of thechamber 525 and no sensing devices within the chamber 525.

Still referring to the exemplary embodiment of FIG. 5, two scales may bedefined on the rotor 505 in order to determine the rotor position to adesired level of resolution. The scales mat be positioned andconstructed to cause variations in the magnetic flux intensitydetermined by sensor 540. The sensor output may then vary according aparticular portion of the scale affecting the sensor, thus providing aposition indicator. For example, a first scale may provide a highresolution incremental position with signal interpolation and a secondscale may provide an absolute position of the rotor 505 within one cycleof the first incremental scale.

An exemplary incremental scale 705 is depicted in FIG. 7. FIG. 7 showssensor systems 720, 725 on one side of a wall 730 of a chamber, forexample chamber 525 that interact with incremental scale 705. Whileshown in this example as a linear scale for simplicity, it should beunderstood that both the incremental and absolute scales discussed abovemay also have rotary configurations. Incremental scale 705 may include aprofile 710 having a regularly spaced tooth pitch 715. Other regularpatterns may be utilized on the incremental scale so long as they aresuitable for indicating incremental positions along the scale.Incremental scale 705 may be machined from a suitable material andrigidly applied to rotor 505. In other embodiments scale 705 may bemolded into, machined into, or otherwise made integral with rotor 505.The sensor systems 720, 725 each include a sensor 740, 755, respectivelyand a magnetic source 745, 765, respectively. Sensors 740, 755 mayprovide an analog or digital output. The sensor systems 720, 725 arepositioned in this embodiment such that a sensor 740 and a magneticsource 745 of a sensor system, for example, sensor system 720, are atthe same position relative to a pitch of the incremental scale 705. Inother words, the center distance 750 between a corresponding sensor 740and magnetic source 745 of the same sensor system 720 may be setapproximately to an integer pitch number 715 of the incremental scale705. Sensor systems 720, 725 may be located a fractional incrementalscale pitch distance 13 from each other, such that their output may befor example, 90 degrees out of phase, due to different air gapreluctance along the sensor path.

In at least one embodiment sensors 740, 755 may provide sine/cosine-likeanalog signals as outputs. In some embodiments a combined output ofsensors 740, 755 may include a quadrature count. As a result, anincremental position may be determined as the result of the quadraturecount of the sine waves plus an interpolated position within aparticular sine cycle. Actual resolution may depend on the number ofbits of an analog to digital converter used to digitize the analogoutputs as well as the noise level present in the outputs. While eachsensor and magnetic source are oriented along a line parallel to thepitch of the scales, or of the pattern used to indicate incrementalpositions along the scales in FIG. 7, other orientations of the sensorsand magnetic sources are also contemplated.

Another exemplary sensor system embodiment is shown in FIG. 8 whichdepicts a sensor system 820 and a rotor 825 having an incremental scale835 positioned in a chamber 830. In FIG. 8, a magnetic source 810 and asensor 815 of the sensor system 820 are oriented along a lineperpendicular to the pitch of the pattern used to indicate incrementalpositions along the scale. Thus, both the sensor and source face thesame portion of the pattern on the scale.

FIGS. 9A and 9B show exemplary embodiments with different incrementalscale locations. In the embodiment of FIG. 9A the incremental scale 905is set off from the rotor 910 and thus is independent from the rotordiameter. In some embodiments, incremental scale 905 may be directlycoupled to rotor 910, for example, by a shaft or other device 915. Inother embodiments, incremental scale 905 may be indirectly coupled torotor 910 using any suitable indirect coupling device or method. In FIG.9B the incremental scale 925 is integrated into an inner diameter of therotor 930. It should be noted that the rotor magnets can be magneticallyisolated from the incremental scale by properly sizing the rotor backingthickness and height.

As mentioned above, two scales may be defined on a rotor to measurepositioning, an incremental scale and an absolute position scale. In atleast one embodiment, the absolute position scale may include additionalposition information required to uniquely locate the rotor position.Absolute position encoders are generally able to provide a uniquelocation without any referencing motion. Typically, such encoders mayrequire several scales, where each scale may be read by an independentsensor system. The number of scales may dictate the number of bits ofthe absolute position encoder and consequently its resolution. In anembodiment using a digital absolute position scale, a digital absoluteposition may be read by a number of independent sensors each facing itsrespective scale. Each sensor may provide the state of one respectivebit of a word that defines a digital position. A classical example of apattern 1005, referred to as a Gray code with 5 bits is shown in FIG.10. Each row of the pattern 1005 includes a 5 bit word that indicates anabsolute position, in this embodiment expressed as an angular positionin degrees. S4 represents the most significant bit of each 5 bit wordand each word differs from the next one by only one single bit, typicalof a Gray code sequence.

An absolute position may be obtained by utilizing a single digital scaleattached to a rotor. In order to read an absolute digital position, aset of sensors may be placed facing the absolute track at a certaininterval relative to each other. The number of sensors may determine thenumber of bits for the absolute position. The use of a single scaledesign is advantageous since it allows for a smaller footprint of thedesign. The bit pattern sequence of the single scale may also have theform of a Gray code, that is, where only one bit changes at a time.

FIG. 11 shows an example of a single scale 1105 for indicating absoluteposition. The single scale 1105 has a pattern that mimics the patternshown for S4 in FIG. 10. By locating five sensors S0 1110, S1 1115, S21120, S3 1125, S4 1130 around the scale 1105 in specific locations, thesensors generate the sequence of FIG. 10 as the pattern rotates, thusgenerating absolute position indications for an attached rotor. It isimportant to understand that a scale may be constructed that utilizesany number of bits suitable for providing a desired positionalresolution. The single absolute scale may be utilized in combinationwith an incremental scale, for example 1135 in FIG. 11 and anincremental scale sensor 1140.

In another embodiment, the single absolute scale 1105 of FIG. 11 may beused alone to simultaneously generate a digital absolute position and aninterpolated incremental position within the resolution of the digitalabsolute position. As mentioned above, the magnetic sensors may becapable of providing digital or analog outputs. In embodiments where themagnetic sensors are capable of providing analog outputs a digitaloutput pattern of an absolute position scale may be generated from theanalog output signals by setting thresholds for determining when a bitof the pattern changes. At the same time the analog value of thechanging signal may be measured and the changing analog values may beutilized to determine a position with additional resolution than thatprovided by the single absolute scale. For example, a digital signalprocessor may be utilized to measure the outputs of the sensors, sensingboth the digital output of the sensors according to the set thresholdsas well as the instantaneous analog output of the sensor that isundergoing a single bit change. This instantaneous analog output may beused to generate the interpolated position between the current digitalabsolute position and the next one.

FIG. 12 illustrates an exemplary change of output by sensor S2 of FIGS.10 and 11 where the rotor is transitioning between 12 and 24 degrees asshown in FIG. 10. In FIG. 12, the interpolated position is representedby the angle θ and the analog sensor output is represented by theparameter V. Since this is a Gray code scale, only sensor S2 is changingits state (from high to low in this case). The interpolated position θcan be determined from the output V as:

$\theta = {\frac{V_{\max} - V}{V_{\max}}\Delta}$Therefore, the total absolute position for rotor given the positionindications of FIG. 12 is:θ_(ABS)=12⁰+θ

The resolution of the interpolated position θ depends on the availableresolution of the conversion function, for example, an A/D converter,used to sample the signal V. One representation for the total number ofbits for expressing the absolute position may be the sum of the numberof sensors plus the number of bits of the AD converter:N _(ABS) =N _(Sensors) +N _(AD)

For example, for the sequence shown in FIG. 10 using the sensors of FIG.11 and a 12 bit AD converter, the total number of bits for expressingthe absolute position would be 17, thus yielding a significantimprovement in resolution over using the five bit Gray code sequence ofFIG. 10 alone.

FIG. 13 shows an exemplary embodiment with multiple scales located onthe same diameter. In this embodiment, an absolute scale 1305, a gapscale 1310 and an incremental scale 1315 are axially offset from eachother. In at least one embodiment the gap scale may be eliminated byrecognizing the upper surfaces 1320 or lower surfaces 1325 of theincremental scale as appropriate as the gap surface and measuring thegap at that point using the reluctance measuring techniques describedherein. In another embodiment, the upper 1330 or lower 1335 surfaces ofthe absolute scale 1305 may also be used to measure the gap using thetechniques described above, eliminating the need for a separate gapscale. In this embodiment, the scales are located on an inner surface ofa rotor 1345 having a number of magnets 1350. A backing 1340 operates toinsulate magnetic sensor systems associated with the scales from theeffects of the rotor magnets 1350.

FIG. 14 shows a multiple sensor system that may utilize an arrangementof multiple scales, for example as shown in FIG. 13. FIG. 14 shows arotor 1405 with a ferromagnetic backing 1410, and one or more permanentmagnets 1415. The rotor may be enclosed within a chamber 1425 capable ofsupporting an environment different from that outside the chamber, forexample a vacuum, high temperature, or corrosive atmosphere. The chamber1425 may be constructed of a nonmagnetic material. The rotor 1405 may bedriven by one or more coils 1420 located outside the chamber 1425.

In this embodiment, three scales are attached or integral to rotor 1405,an absolute scale 1430, a gap scale 1435, and an incremental scale 1440.One or more sensor systems may be associated with each scale. Thisembodiment includes an absolute sensor system 1445 for reading theabsolute scale 1430, a gap sensor system 1450 for reading the gap scale1435, and an incremental sensor system 1455 for reading the incrementalscale 1440. Each of the sensor systems 1445, 1450, 1455 may include anynumber of sources and sensors as described above. As mentioned above,the gap scale 1435 may be combined or superimposed upon any of the otherscales. When combined or superimposed, the gap scale may continue to beread using the gap sensor system 1450 or may be read by the sensorsystem for the scale with which it is combined or superimposed. Itshould be understood that while this embodiment shows three scales andthree sensor systems, that any suitable number of scales and sensorsystems may be utilized.

In this embodiment, the multiple sensor system also may includecircuitry 1460 coupled to the absolute, incremental, and gap sensorsystems. The circuitry may provide an output indicative of a measuredposition of the reactive motor element from a combination of outputs ofthe absolute, incremental, and gap sensor systems.

FIG. 15 shows an exemplary sensor system 1500 suitable for use with theembodiments described herein. Sensor system 1500 may utilize magneticcircuit principles, for example, similar to those described above todetermine a distance from a ferromagnetic target 1555, for example, arotor backing to the sensor system's reference frame. The ferromagnetictarget 1555 may be a flat or curved surface or have any machined profileattached to, embedded in, or otherwise integral to the target, forexample, the scales discussed above. The sensor system 1500 may includea ferromagnetic element 1505, a magnetic source 1510, for example, apermanent magnet, a number of magnetic sensors 1515, 1520, 1525, 1530and conditioning circuitry 1535. The ferromagnetic element 1505 maycircumscribe the magnetic source 1510. In other embodiments, theferromagnetic element 1505 may surround or even enclose the magneticsource 1510. In at least one exemplary embodiment, the ferromagneticelement 1505 may have a cup shape with a closed end 1565 and an open end1570. The magnetic source 1510 may have a cylindrical shape where thedirection of magnetization is parallel to the axis of symmetry of theferromagnetic element 1505. The magnetic source 1510 may be a permanentmagnet, an electromagnet, or any other suitable source of magneticenergy. The magnetic source 1510 may be attached within theferromagnetic element to the center of the ferromagnetic element 1505 byattractive forces and may be held in place using a suitable fastener,for example an adhesive. In at least one embodiment, the sensor system1500 may be oriented such that the open face 1570 of the cup faces theferromagnetic target 1555.

The embodiment shown in FIG. 15 establishes a magnetic circuit betweenthe ferromagnetic element 1505 and the magnetic source 1510 such thatthe flux density is symmetric about the axis of the cup or anyconcentric perimeter between the magnetic source 1510 and theferromagnetic element 1505. The shape of the ferromagnetic element 1505influences the shape of the magnetic field. In embodiments where theferromagnetic element 1505 is cup shaped, the magnetic field isrelatively confined, resulting in an increased sensitivity to variationsin the distance 1560 to the ferromagnetic target. The ferromagneticelement 1505 may have a shape tailored to create a specifically shapedmagnetic field. In some embodiments the ferromagnetic element 1505 mayalso be fashioned to provide a specific sensitivity to distancevariations between the sensor system 1500 and the ferromagnetic target1555.

Magnetic sensors 1515, 1520, 1525, 1530 may operate to sense the fluxdensity and may be located in an orbital configuration at a constantradial distance from the axis of symmetry of the ferromagnetic element1505. The magnetic sensors may also be positioned such that theiroutputs are approximately the same. While four magnetic sensors areshown, it should be understood that any suitable number of magneticsensors may be utilized. Outputs of the magnetic sensors 1515, 1520,1525, 1530 may be provided to the conditioning circuitry 1535.Conditioning circuitry 1535 may include signal processing circuitry forprocessing the sensor outputs, for example, to provide compensation,filtering, noise reduction, or any other suitable signal processing. Thesensor output signals may generally be processed to provide a sensorsystem output 1550. The use of additional sensors may improve the noiseimmunity of the system. The ferromagnetic element 1505 may also operateas a magnetic isolation cage for the magnetic sensors minimizingexternal magnetic interference from the surrounding environment. Thesensor system 1500 is thus configured to measure alterations in themagnetic flux density vector detected by the magnetic sensors. Inparticular, the sensor system 1500 may measure alterations in themagnetic flux density vector due to the presence of the ferromagnetictarget 1555. In at least one embodiment, outputs of the magnetic sensors1515, 1520, 1525, 1530 may be conditioned to provide a sensor systemoutput 1550 indicating the distance 1560 to the ferromagnetic target1555.

FIG. 16 shows an exemplary arrangement of magnetic sensors around theferromagnetic element. In this embodiment magnetic sensors may bearranged in pairs 1610 1615, 1620 1625, 1630 1635, 1640 1645 withalternating orientations relative to the flux density lines between theferromagnetic element 1505 and the magnetic source 1510. In thisembodiment, each sensor pair may provide a differential output. Summing1650 and differential conditioning 1655 circuitry may be part ofconditioning circuitry 1535 and may further provide sensor system output1550 as a differential signal. The use of differential outputs mayimprove noise immunity, in particular where signals have low levels, aresubject to a hostile electrical electromagnetic environment, or travelany appreciable distance. For example, providing sensor system output1550 as a differential signal may improve noise immunity as the outputis provided to reading device 1660.

In other embodiments, the magnetic sensors do not have to be placed atequal radial distance from the axis of symmetry and that their outputsneed not be necessarily equal and yet the outputs can be suitablyprocessed to yield the effective target distance. It should beunderstood that any number of magnetic sensors may be used, eitherungrouped, or grouped together in any suitable number or arrangement.

In addition to measuring target distance, the sensing system 1500 mayalso be used interchangeably with sensing systems 720 or 725 or 820 inFIGS. 7 and 8, to read incremental or absolute position tracks.

Returning to FIG. 15, the ferromagnetic target 1555, once located infront of the sensor system 1500 alters the magnetic flux density vectordetected by magnetic sensors 1515, 1520, 1525, 1530, thus affectingoutput signal 1550. The distance 1560 between the target 1555 and thesensor system may determine the value of sensor system output 1550. Thesensor system output 1550 may vary according to any magnetic fluxvariations introduced by one or more scales that may be attached to orintegral with ferromagnetic target 1555.

The shape of the magnetic source 1510 and the ferromagnetic element 1505may be modified to obtain a particular flux density pattern orconfiguration, or to optimize or otherwise improve the sensor systemoutput 1550 or the distance 1560. For example, in some embodiments, atleast one of the ferromagnetic element 1505 and the magnetic source 1510may have the shape of a cylinder, cone, cube or other polyhedron,paraboloid, or any other suitable shape. As mentioned above, any numberof sensors may be utilized. Furthermore, the sensors may have anysuitable arrangement for obtaining a particular flux density pattern, orfor optimizing the sensor system output 1550 or the distance 1560.

The sensor system 1500 is suitable for use in the embodiments describedherein, for example, through a wall of non-magnetic material as used inthe chambers disclosed herein that may isolate a target rotor or scalefrom the sensor system. The sensor system 1500 is suitable for use invacuum automation system embodiments The sensor system 1500 isparticularly suited for measuring magnetic flux, gaps and scales for allthe embodiments described herein.

FIG. 17 illustrates an exemplary motor 2110 including a positionfeedback system 2100 in accordance with an exemplary embodiment.Although the embodiments disclosed will be described with reference tothe embodiments shown in the drawings, it should be understood that theembodiments disclosed can be embodied in many alternate forms ofembodiments. In addition, any suitable size, shape or type of elementsor materials could be used.

The feedback system of the exemplary embodiments may provide highresolution positional feedback for any suitable motor. The feedbacksystem of the exemplary embodiments may allow for the simultaneousmeasurements of eccentricity and orientation (e.g. rotation) withrespect to a stator of the motor based on tangential positionalmeasurements.

The motor 2110 shown in FIG. 17 includes a single rotor/stator forexemplary purposes only and it should be realized that the motor 2110may include any suitable number of rotors arranged in any suitableconfiguration including, but not limited to, coaxial and non-coaxialconfigurations. In the exemplary embodiment of FIG. 17, the stator 2110Smay be, for example, an iron-core stator but in alternate embodimentsthe stator may be any suitable stator. The rotor 2110R may be, forexample, constructed of any suitable material and include permanentmagnets 2110M and iron backings 2110B. In alternate embodiments, therotor may include any ferromagnetic material for interacting with thestator 2110S.

The stator 2110S may include any suitable winding sets for controllingthe position of the rotor 2110R in, for example, the X-Y plane and/or inthe Z-direction. In alternate embodiments the winding sets may have anysuitable configuration. The interaction between the stator 2110S and therotor magnets 2110M may produce forces that passively levitate the rotor2110R. The levitation force may be a result of curved magnetic fluxlines which in turn may be generated by, for example, an offset of anedge of the rotor magnet relative to the an edge of the stator asdescribed in U.S. Provisional Patent application 60/946,687, entitled“ROBOT DRIVE WITH MAGNETIC SPINDLE BEARINGS”, filed 27 Jun. 2007, thedisclosure of which is incorporated by reference herein in its entirety.In alternate embodiments the levitational forces may be generated in anysuitable manner.

The feedback system 2100 of the exemplary embodiment includes multipleread heads 2130 and a scale 2120. The read heads 2130 may be anysuitable read heads including, but not limited to non-contact optical,capacitive, inductive and magnetic read heads. In alternate embodimentsthe read heads may be contact based read heads. The read heads may belocated at any suitable location in the motor such that the read heads2130 are fixed with respect to the stator 2110S. In alternateembodiments the read heads 2130 may have any suitable relationship withrespect to the stator 2110S. As may be realized in alternate embodimentsthe read heads 2130 may be positioned, configured and/or suitablyisolated from the rotor 2110R and stator 2110S such that, for example,magnetic interaction between the read heads 2130 and the rotor 2110R andstator 2110S does not alter the readings provided by the read heads2130.

The read heads 2130 may be communicably coupled to any suitableprocessor 2160 configured to receive output signals from the read heads2130 and process those signals as will be described below to determinethe positional data with respect to the rotor 2110R. For exemplarypurposes only, the read heads 2130 may be in communication with theprocessor 2160 through any suitable wired or wireless connectionsincluding, but not limited to, wide area networks, local area networks,Bluetooth, infrared, radio frequency or any other suitable connections.In one or more embodiments, the read heads 2130 may include one or moresensing mechanisms 500 or sensor systems 1500 described above.

The scale 2120 may be any suitable scale including, but not limited to,absolute or incremental scales configured for use with the read headsdescribed above. It is noted that while one scale is shown in theFigures that in alternate embodiments any suitable number of scales maybe used. As a non-limiting example, in one alternate embodiment, eachread head 2130 may have its own respective scale while in otheralternate embodiments some read heads may share one scale while otherread heads share a different scale.

In one exemplary embodiment, the scale 2120 may be bonded to orotherwise attached to the rotor 2110R. In other exemplary embodimentsthe scale 2120 may be embedded in the rotor 2110R such as by, machining,etching or any other suitable manufacturing technique. In alternateembodiments the scale 2120 may be a disk attached to and extendingradially from the rotor. In other alternate embodiments the scale mayhave any suitable configuration. The scale 2120 may be configured suchthat the graduations 2120G on the scale are arranged so the read headscan effect the detection of eccentricity and/or rotation of the rotor110R as will be described in greater detail below. In alternateembodiments the graduations on the scale may be arranged in any suitablemanner.

Still referring to FIG. 17, it is further noted that the feedback system2100 of the exemplary embodiments may be utilized in any suitableenvironment including, but not limited to, vacuum, atmospheric orcontrolled air environments. In one exemplary embodiment, the motor mayinclude a boundary 2140 that may allow the rotor 2110R to operate in avacuum while the stator 2110S operates in an atmospheric environment. Inalternate embodiments each of the stator and rotor may operate in anysuitable environment that may be the same or different from each other.The boundary 2140 may be constructed of any suitable material for usein, for example, a vacuum environment and from material that can beinterposed within magnetic fields without causing a flux short circuitor being susceptible to eddy currents and heating from magneticinteraction. The boundary may also be coupled to suitable heat transferdevices (e.g. passive or active) to minimize temperatures in the drivesection. In one exemplary embodiment where the read heads 2130 areoptical read heads 2130 the boundary may include optical view ports toallow the read heads 2130 to read the scale 2120. Where the read heads2130 are capacitive, inductive or magnetic (e.g. Hall sensors) there maynot be any view ports associated with the read heads 2130.

Referring now to FIG. 18, a schematic view of the feedback system 2100′is shown in accordance with an exemplary embodiment. In the exemplaryembodiment shown in FIG. 18, the feedback system 2100′ includes threeread heads 2130A-2130C but in alternate embodiments the feedback system2100′ may have more or less than three read heads. The read heads2130A-2130C are shown in the Figure as being arranged around the stator2110S in a substantially equally spaced manner such that the read headspoint radially at the scale 2120. In alternate embodiments, the readheads 2130A-2130C may be arranged around the stator 2110S with anysuitable predetermined spacing arrangement and have any suitableorientation with respect to the scale 2120. In one exemplary embodiment,each of the read heads 2130A-2130C may be configured to provide positioninformation corresponding to the distance (e.g. dA, dB, dC) between thepoint on the scale 2120 that a respective read head is viewing and anorigin SO of the scale 2120. This information, for example, may be usedto determine the eccentricity and orientation of the rotor 2110R withrespect to the stator 2110S. In alternate embodiments the read heads2130A-2130C may provide any suitable information for determining theeccentricity and orientation of the rotor 2110R with respect to thestator 2110S. It is noted that the distances dA, dB, dC shown in FIG. 18are shown extending in a clockwise direction but in alternateembodiments the distance may be in a counterclockwise directiondepending on, for example, the direction of rotation of the rotor 2110R.

Referring now to FIG. 19 the determination of the eccentricity andorientation of the rotor 2110R using tangential position measurementsfrom four read heads 2230A-2230D will be described in accordance with anexemplary embodiment. However, it is noted that the exemplary equationsdescribed below corresponding to the four read heads 2230A-2230D may beadapted for any suitable number of read heads such that the eccentricityand rotational position of the rotor 2110R may be determined.

As may be realized, during the operation of the motor 2110 the rotor2110R may deviate from a first center of rotation C to a second centerof rotation C1. This deviation may be due to, for example, radial and/oraxial loads applied to the rotor. The feedback system 2100″ may beconfigured to calculate the deviation as well as the rotationalorientation of the rotor 2110R. In the exemplary positionaldetermination described below it is assumed that the distances d1-d4increase as the rotor 2110R turns in a counterclockwise direction.However, in alternate embodiments it may be assumed that the distancesd1-d4 increase as the rotor 2110R turns in a clockwise direction whereappropriate changes are made to the equations described below.

As a non-limiting example, in this exemplary embodiment the eccentricityor deviation from the center point C can be found using the followingequations:x ₀ =r cos [(d ₂ −d ₄)/(2r)]  (100)y ₀ =r cos [(d ₃ −d ₁)/(2r)]  (101)where x₀ and y₀ respectively denote the x and y components of theeccentricity of the rotor 211R. As may be realized from the aboveequations the eccentric distance x₀ may be found using an anglecorresponding to the arc length 2240X as tangentially measured by theread heads 2230D and 2230B. Similarly the eccentric distance y₀ may befound using an angle corresponding to the arc length 2240Y astangentially measured by the read heads 2230C and 2230A. The rotationalorientation or position of the rotor can be found using the followingequations:

$\begin{matrix}{\theta_{1} = {{d_{1}/r} - {{a\sin}\left( {y_{0}/r} \right)}}} & (102) \\{\theta_{2} = {{d_{2}/r} - {3{\pi/2}} + {{a\sin}\left( {x_{0}/r} \right)}}} & (103) \\{\theta_{3} = {{d_{3}/r} - \pi + {{a\sin}\left( {y_{0}/r} \right)}}} & (104) \\{\theta_{4} = {{d_{4}/r} - {\pi/2} - {{a\sin}\left( {x_{0}/r} \right)}}} & (105) \\{\theta_{0} = {{\sum\limits_{i = 1}^{4}{\theta_{i}/4}} = {{\left( {d_{1} + d_{2} + d_{3} + d_{4}} \right)/\left( {4r} \right)} - {3{\pi/4}}}}} & (106)\end{matrix}$where θ₀ is the orientation of the rotor 2110R. θ₁-θ₄ respectivelydenote the angle between the read heads 2230A-2230D and the origin SO ofthe scale 2120. The distances between the scale origin SO and the readheads 2230A-2230D are respectfully denoted as d₁-d₄. The radius of thescale 2120 is denoted by the indicator r. The above equations mayprovide a substantially exact determination of the position (i.e.eccentricity) of the rotor 2110R in the X-Y plane and the rotationalorientation θ₀ of the rotor 2110R with respect to any desired referencepoint.

In another example, approximations of the eccentricity and rotationalorientation θ0 of the rotor may also be determined without evaluatingtrigonometric functions. The positional approximations can be determinedusing the following equations:

$\begin{matrix}{x_{0} = {{{- \left( {d_{2} - d_{4} - {\pi\; r}} \right)}/2} = {\left( {d_{4} - d_{2} + {\pi\; r}} \right)/2}}} & (107) \\{y_{0} = {{{- \left( {d_{3} - d_{1} - {\pi\; r}} \right)}/2} = {\left( {d_{1} - d_{3} + {\pi\; r}} \right)/2}}} & (108) \\{\theta_{1} = {\left( {d_{1} - y_{0}} \right)/r}} & (109) \\{\theta_{2} = {\left( {d_{2} - {3\pi\;{r/2}} + x_{0}} \right)/r}} & (110) \\{\theta_{3} = {\left( {d_{3} - {\pi\; r} + y_{0}} \right)/r}} & (111) \\{\theta_{4} = {\left( {d_{4} - {\pi\;{r/2}} - x_{0}} \right)/r}} & (112) \\{\theta_{0} = {{\sum\limits_{i = 1}^{4}{\theta_{i}/4}} = {{\left( {d_{1} + d_{2} + d_{3} + d_{4}} \right)/\left( {4r} \right)} - {3{\pi/4}}}}} & (113)\end{matrix}$where θ₀, θ₁-θ₄, d₁-d₄ and r denote the same features as describedabove.

It should be realized that the above solutions for determining theeccentricity (i.e. x₀ and y₀) and rotational orientation (i.e. θ₀) ofthe rotor 2110R are for exemplary purposes only and that other solutionsfor determining the eccentricity and rotational orientation usingtangential position measurements may be used.

Turning now to FIG. 20A, another embodiment may provide measurement ofan X-Y displacement of a rotor using a plurality of sensor sets, in thisexample, two sets of sensors. FIG. 20A shows an exemplary embodimentincluding a portion of a motor 2000 having a stator 2003 with a statorbacking 2005 and one or more windings 2010, a rotor 2015, and at leasttwo sensor pairs 2020, 2025. The rotor 2015 may include a rotor backing2030 on which may be mounted a number of rotor magnets 2035.

In this embodiment, sets of sensors are used to detect displacement ofthe rotor 2015 with respect to the stator 2003, in particular adisplacement along the gap 2040 between the stator 2003 and the rotor2015. For example, a first set of sensors 2020 measures the displacementat a first location Y while a second set of sensors 2025 measures thedisplacement at a second location X at an angular offset A from thefirst location Y. While in this embodiment, the locations are offset by90 degrees, it should be understood that any suitable angular offset mayutilized. Each sensor set may include two sensor systems, X1 X2 insensor set 2025 and Y1 Y2 in sensor set 2020. Each sensor set mayinclude additional sensor systems in other embodiments. Each sensorsystem may be similar to sensor system 1500 in FIG. 15. The sensorsystems in each set generally have magnetic sources with opposingpolarities.

As shown in FIG. 20B, sensor systems X1 (or Y1) includes a ferromagneticelement 2040 and a magnetic source 2045 and sensor systems X2 (or Y2)includes a ferromagnetic element 2050 and a magnetic source 2055.Magnetic sources 2045 and 2055 are positioned in opposing polarity. Asan example, and as shown in FIG. 20B, sensor systems X1 and Y1 have theNorth pole N of the magnetic source 2045 facing inward of theferromagnetic element 2040, while sensor systems X2 and Y2 have theSouth pole S of the magnetic source 2055 facing inward of theferromagnetic element 2050. Extraneous magnetic fields such as thosecaused by eddy currents have opposite effects on each sensor systemwithin each pair of sensor systems. Thus, the effects of such extraneousfields may be eliminated by taking an average of the outputs of eachsensor system within each pair. Noise attenuation may be advantageouslyimproved using these arrangements and techniques.

FIG. 21 shows an embodiment similar to the embodiment shown in FIG. 7where sensor systems are used to read an incremental track to producesine and cosine signals. The embodiment of FIG. 21 may use 4 sensorsystems. Each sensor system may be similar to sensor system 1500 in FIG.15. The sensor systems of FIG. 21 may be positioned to produce sine andcosine signals with no DC offset and an amplitude that is invariant tosmall changes in displacement along the Z axis and the gap separatingthe sensor systems and the incremental track.

In FIG. 21, sensor systems 2101 2102 2103 2104 are positioned along anincremental scale 2105 which may have a regularly spaced tooth pitch2110 and an area with a flat face 2115. In this embodiment, sensorsystems 2101 and 2102 are positioned as a first pair along the scale2105 to output a sine signal and sensor systems 2103 and 2104 arepositioned as a second pair along the scale 2105 to output a cosinesignal. Sensor systems within each pair may be offset by 180 degreeswhile corresponding sensor systems in the first pair are offset fromcorresponding sensor systems in the second pair by 90 degrees. Eachsensor system 2101 2102 2103 2104 may have at least two magnetic sensorsA and B. The A sensors may be positioned to read the tooth profile ofthe incremental scale 2105. A change in displacement along the Z axisand along the gap of the A sensors or the scale 2105 will generallyaffect both the signal amplitude and DC offset of the A sensors. The Bsensors may be positioned to read only the flat face area 2115 of thescale 2105. As a result, signals output by the B sensors may not beaffected by displacement along the Z axis, but may only be affected bychanges along the gap 2120.

By combining signals from the A and B sensors, a sine or cosine signalthat is invariant to changes in displacement along the gap may beobtained. In addition, the outputs of sensors within each pair of sensorsystems may be 180 degrees out of phase and thus may vary in the samedirection as a result of any displacement along the Z axis. By combiningthe signals from sensors A and B within each pair of sensor systems, asine or cosine signal that is invariant to displacement along the Z axisand the gap 2120, with no DC offset may be obtained.

The disclosed embodiments provide techniques for determining arotational position of a rotor without invading an isolated environmentin which the rotor may operate, without requiring electronics or sensorswithin the isolated environment. In one embodiment, a single scale maybe used to determine both an absolute and an incremental position.

The presently disclosed embodiments also provides a sensor system with aunique arrangement of a ferromagnetic element a magnetic source andmagnetic sensors that generates uniform magnetic flux density lines suchthat the sensors may be placed in a orbital configuration around themagnetic source.

The disclosed embodiments also provide a feedback system for a motorthat includes a unique structure and technique for determiningeccentricity and rotational position of a rotor of the motor.

It should be understood that the foregoing description is onlyillustrative of the present embodiments. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the embodiments disclosed herein. Accordingly, theembodiments are intended to embrace all such alternatives, modificationsand variances which fall within the scope of the appended claims.

1. A sensor system comprising: a magnetic source; a ferromagneticelement circumscribing the magnetic source; a plurality of magneticsensors arranged around an axis of symmetry of the ferromagnetic elementwhere the plurality of magnetic sensors are disposed in an area betweenthe magnetic source and the ferromagnetic element, wherein the magneticsource is located so that a direction of magnetization is parallel tothe axis of symmetry of the ferromagnetic element, and wherein an openend of the ferromagnetic element faces a ferromagnetic target formeasuring a position of the ferromagnetic target.
 2. The sensor systemof claim 1, wherein the ferromagnetic element has a cup shape.
 3. Thesensor system of claim 1, wherein the magnetic sensors are arranged inpairs, with each pair member having alternating orientations relative toflux density lines between the ferromagnetic element and the magneticsource, wherein each sensor pair is configured to provide a differentialoutput having at least an immunity to noise.
 4. The sensor system ofclaim 1, further comprising a first scale coupled to the ferromagnetictarget indicating an absolute position of the ferromagnetic target. 5.The sensor system of claim 4, wherein the magnetic sensors are operableto sense variable magnetic flux intensities caused by the first scaleand to output a signal indicating a measured absolute position of theferromagnetic target.